In general, a wide measuring range is required for a measuring system to measure the neutron flux level in a pressure vessel in a reactor. To measure a neutron flux level in a boiling water reactor (BWR), for example, the required measuring range is 11 decades. This implies that it is very difficult to obtain a satisfactory result by the use of a single measuring device. Conventionally, three measuring devices are combined to form a measuring system for neutron flux level measurement. Of these measuring devices, the first one is for low neutron flux level measurement. In the start-up range of the reactor operation, the neutron flux level is low and the reactor power is proportional to the counting rate of a neutron counter. The counting rate of the output pulses from a neutron counter capable of producing the pulse output up to 6 decades is sufficient for the neutron flux level measurement in this range. When the reactor operation enters the intermediate range between the start-up range and the power range, the neutron flux level increases to a high level. In this condition, the counting rate measuring method is insufficient for the reactor power measurement in this intermediate range. However, Compbell's theory is applicable for the reactor output measurement in this intermediate range, since the reactor output is proportional to a mean squared value of an AC component contained in the output signal of the neutron detector. Accordingly, in this range the neutron flux level can be measured using a measuring method based on Campbell's theory. In the power range, the reactor output is proportional to a DC component in the detector output signal. Accordingly, the reactor power in this range can be measured by measuring this DC component.
For measuring the reactor power of the BWR, for example, 4 neutron detectors are installed for the start-up range; 8 detectors for the intermediate range; and 100 to 200 detectors for the power range. The types of neutron detectors are different for each reactor operation range.
The measuring devices for the respective operation ranges will now be described in detail. FIG. 1 shows a neutron flux level measuring device for the start-up range. In FIG. 1, the output pulses of a neutron counter 1 installed in the pressure vessel are transferred to an input terminal of an input circuit 4 for a preamplifier 3 for pulse amplification, through a coaxial cable 2 which is 10 m to 20 m long. The input circuit 4 is a low input impedance circuit containing a capacitor C1 and a resistor R1. The input circuit 4 is impedance-matched to the coaxial cable 2 so as to prevent a signal reflection on the signal path therebetween. In FIG. 1, CO represents a capacitance of the coaxial cable 2, HV a high tension voltage terminal, and RH a resistor inserted between the high tension voltage terminal HV and the input terminal of the input circuit 4.
FIG. 2 shows a prior art flux level measuring device for intermediate range measurement. In the figure, a neutron detector 5 for the intermediate range is coupled to an input terminal of an input circuit 7 for a preamplifier 6 for voltage amplification. The preamplifier 6 is of the low noise type, which amplifies an AC component output (Campbell signal) from the neutron detector 5. The input impedance of the input circuit 7 is selected to be from 5 kiloohms to 10 kiloohms.
As already mentioned, it is technically very difficult to measure both the pulse signal and the Campbell signal by the use of the same measuring device. Specifically, it is very difficult to design a measuring device in which a pulse channel preamplifier and a Campbell channel preamplifier are connected to the same type of neutron detector for separating the signals from the neutron detector into a pulse signal and a Campbell signal and amplifying the corresponding signal so as to satisfy a required condition. It is also difficult to measure the reactor power so that the measuring range of the measuring device of FIG. 1 partially overlaps that of the measuring device of FIG. 2.
A neutron flux measuring device shown in FIG. 3 is so designed as to amplify a pulse signal and a Campbell signal from a wide range neutron detector 8 by using a single preamplifier 9. The preamplifier 9 which is provided with a low input impedance circuit 10, including a capacitor C3 and a resistor R3, has low noise performance and a wide frequency band. In the measuring device shown in FIG. 3, a noise voltage e.sub.n (r.m.s.) which is converted into an input signal to the preamplifier 9 is expressed by equation (1) ##EQU1## where Rin is resistance of the resistor R3, k a Boltzman constant, 1.3804.times.10.sup.-25 Joul/.degree.K, T an absolute temperature (.degree.K), and B a frequency band width (Hz) of the preamplifier 9. For the noise voltage e.sub.n (r.m.s) as given above, a noise current i.sub.n (r.m.s), which is converted into an input current to the preamplifier 9 or an equivalent input noise current, is given by ##EQU2## Incidentally, the noise current is generated as a thermal noise from the resistor R3. The Campbell signal derived from the detector 8 is a current signal. Accordingly, if the Campbell signal from the wide range neutron detector 8 is smaller than the equivalent input noise current i.sub.n (r.m.s) as given by equation (2), it is impossible to detect the Campbell signal. Particularly, since the resistance Rin of the input resistor R3 of the preamplifier 9 is small (50 ohms or so), the equivalent input noise current i.sub.n (r.m.s) is extremely large. In the measurement of the Campbell signal using the measuring device shown in FIG. 3, the signal to noise ratio S/N is approximately 200 times that when the preamplifier 6 of FIG. 2, which has the input impedance circuit 10 including the resistor of 10 kiloohms, is used for measuring the same Campbell signal. For this reason, it is very difficult to amplify both the pulse signal and the Campbell signal by the single preamplifier 9 with a low input impedance.
A measuring device shown in FIG. 4 is designed to measure a pulse signal and a Campbell signal by using one neutron detector, and is disclosed in Kokai (Japanese Unexamined patent publication) No. 56-117193. In the measuring device of FIG. 4, the neutron flux level signal detected by a fission counter 11 is applied through the coaxial cable 2 to a preamplifier 13 with an input impedance circuit 12 which is made up of a capacitor C4 and a resistor R4, and to a preamplifier 15 with an input impedance circuit 14 which is made up of a capacitor C5 and a resistor R5. The preamplifier 13 amplifies a high frequency band signal which is derived from the fission counter 11 and delivered through the input impedance circuit 12. The preamplifier 15 amplifies the intermediate frequency band signal which is derived from the fission counter 11 and delivered through the input impedance circuit 14. Thus, the preamplifier 13 is a pulse amplifier for amplifying a pulse signal, while the preamplifier 15 is a Campbell signal amplifier for amplifying a Campbell signal.
In FIG. 4, the neutron flux level signal from the fission counter or the neutron detector 11 is routed to the input impedance circuit 12 or the input impedance circuit 14 depending on the frequency band of the signal. The preamplifier 13 amplifies only the high frequency component applied through the input impedance circuit 12. The preamplifier 15 amplifies only the intermediate frequency component, i.e., the Campbell signal, which is applied thereto through the input impedance circuit 14. Accordingly, the amplifiers 13 and 15 can amplify the neutron flux level signal with an improved S/N. The amplifiers 13 and 15 have frequency characteristics as shown in FIG. 5. In the graph, the x-distance represents frequency (Hz) and the y-distance an amplification factor (dB). In FIG. 5, f.sub.CO, f.sub.CL and f.sub.CH indicate respectively the center frequency, the lower limit frequency and the upper limit frequency of the Campbell signals. f.sub.PO, f.sub.PL and f.sub.PH respectively indicate the center frequency, the lower limit frequency and the upper limit frequency of the pulse signals. If the capacitance C4' of the capacitor C4 and the resistance R4' of the resistor R4 in the input impedance circuit 12 are selected so as to satisfy the following relations ##EQU3## the pulse signal component can be smoothly supplied to the preamplifier 13. If the capacitance C5' of the capacitor C5 and the resistance R5' of the resistor R5 of the input impedance circuit 14 are selected so as to satisfy the following relations ##EQU4## the Campbell signal component can be smoothly sent to the preamplifier 15. The noise applied from the preamplifier 13 to the preamplifier 15 can be removed by properly selecting the frequency characteristic of the preamplifier 15.
A serious disadvantage of the measuring device shown in FIG. 4 is that it has a long response time. The response time means the time taken for the signal generated in the neutron detector 11 to be detected by a measuring system connected to the outputs of the preamplifiers 13 and 15.
The response time of the measuring device of FIG. 4 will be described in detail referring to FIG. 6. The Campbell signal from the neutron detector 11 includes a fluctuation component .sigma..sub.s with respect to the level S of the Campbell signal. The fluctuation component is represented by a mean squared value of the amplitude of the Campbell signal. The ratio of the fluctuation component .sigma..sub.s to the Campbell signal level S, viz. .sigma..sub.s /S, is called a fluctuation factor, here denoted as I', and is given as ##EQU5## where .tau. is a response time, and Nn is a pulse rate (c.p.s) of the pulse signals derived from the neutron counter 11. For f.sub.CL and f.sub.CH, see FIG. 5. When f.sub.CL =1 kHz and f.sub.CH =10 kHz, and .tau. is 1 msec, 10 msec, 0.1 sec or 1 sec, the fluctuation factor I' of the Campbell signal versus the pulse rate Nn (c.p.s) is as shown in FIG. 6. As seen from FIG. 6, for measuring the neutron flux level in the intermediate range while keeping the fluctuation factor I' below approximately 1%, the response time must be 0.1 sec or more. However, in the case where the response time is 0.1 sec or more, the measuring devices can not effectively measure the neutron flux level within a very short response time when the reactor power abruptly increases and the pulse rate Nn is very large.
The disadvantages of the prior measuring devices by which the pulse signal and the Campbell signal are measured by one neutron detector, will again be described as follows. If the measuring device shown in FIG. 3 is used for such a measurement, the S/N ratio is deteriorated in the measuring range of the Campbell signal. This leads to a narrow overlapping range of the Campbell signal measuring range and the pulse signal measuring range. As a result, the reliability of the measurement is deteriorated. In the measuring device shown in FIG. 4, the response time .tau. in the measuring range of the Campbell signals is made long. Therefore, the prior measuring device shown in FIG. 4 can not measure the neutron flux level when the neutron detector 11 produces pulse signals at high pulse rate and the measurement must be done within a short response time.